1. Field of the Invention
The invention relates to a process for the isolation of noble metals from ores or other materials containing the latter by means of a leaching process carried out under acid conditions in the presence of specified thioureas.
2. Prior Art Statement
Nowadays, noble metals are generally isolated by treating their ores with alkaline cyanide solutions, atmospheric oxygen being introduced as an oxidizing agent. Although this method of treatment is used all over the world, it suffers from a number of substantial disadvantages: the highly toxic cyanides constitute an obvious source of danger. The use of dilute cyanide solutions requires large volumes and long reaction times, in some cases extending over 24 hours, which has an unfavorable effect on the production costs of the noble metal. In addition, many ores are cyanide-resistant, that is to say the noble metals present are only dissolved out to a very low and insignificant extent.
Recently, the use of thiourea as a complex-former in the isolation of noble metals has been publicized increasingly, see T. Groenewald, Hydrometallurgy 1 (1976) 277-290; J. B. Hiskey, Proceedings from the 110th AIME Meeting, Chicago, 22-26 (1981), page 83. Such processes are superior in many respects to the processes operating with cyanides, since, for example, the treatment times are substantially shorter and the noble metals can be isolated successfully even from cyanide-resistant ores. The disadvantage is the high consumption of thiourea, resulting from oxidation. In addition, the sulphur which is precipitated renders the material to be leached passive and prevents the achievement of quantitative yields of noble metal.
There has been no lack of attempts to reduce sensitivity of thiourea to oxidation by substitution, without thereby impairing its ability to form complexes with noble metals. Success in this direction has not been achieved, since either the solubility of the reagent has been lost as a result of the substituents, or its capacity to form complexes has no longer been adequate.
The object was therefore to develop a process which does not have the disadvantages described above, but which makes it possible to carry out, at high cost-efficiency, the extraction of noble metals from their ores or from materials containing them.
In this respect, in the present invention, it has been found, surprisingly, that it is possible, by means of cyclic thiourea derivatives, to isolate the noble metals from their ores or the materials accompanying them in a high yield and within very short leaching times.